mgw: Support receiving lowercase LCO codec

MGCP RFC3435 (https://tools.ietf.org/html/rfc3435) states almost all
text has to be handled in a case-insensitive way, except SDP parts.

Related: OS#4001
Change-Id: I51dc1cdcbe2a5587769335fbecb5039ef22cae5d
4 files changed